1. Field of the Invention
The present invention relates generally to photonics assemblies, and more specifically to a monolithically integrated alignment photonics assembly for actively coupling optical energy between optical devices.
2. Description of the Prior Art
Compact and simple photonics systems are essential in optical communication applications. Photonics systems require high light transmission efficiencies in order to obtain low error rates. The transmission efficiency is measured as insertion loss for photonics applications and becomes more important for photonics systems working at high data transmission rates. The high data transmission (high bandwidth) rates require the use of single mode and polarization maintaining fiber optics where optical alignment from fiber to fiber, transmitter to fiber, transmitter to modulator, transmitter to multiplexer and fiber to receiver becomes critical to minimizing insertion loss. Optical alignment requirements for single mode fibers are at micron and sub-micron levels as opposed to supermicron levels for lower bandwidth multimode fibers. Optical alignment methods are near the limit of improvement using conventional alignment techniques. For example, single mode fiber connectors using actively aligned ferrules, like that described in the publication "Packaging Technology for a 10-Gb/a Photoreceiver Module", by Oikawa et al., Journal of Lightwave Technology Vol. 12 No. 2 pp.343-352, February 1994, are typically limited to 0.2 dB insertion loss. The Okiwawa publication discloses an optical coupling system, illustrated in FIG. 1, containing a slant-ended fiber 46 secured in a fiber ferrule 48 where the fiber ferrule 48 is welded to a side wall 50 of a flat package 52 and a microlens 54 is monolithically fabricated on a photodiode 56 where the photodiode 56 is flip-chip bonded to the flat package 52. An optical signal 58 enters horizontally and is reflected vertically at the fiber's 46 slant-edge. The microlens 54 then focuses the optical signal 58 on the photodiode's 56 photosensitive area.
As described in the Oikwawa publication, maintaining alignment between the fiber and the photodiode chip is essential for optimal coupling of the optical signal. Misalignment can occur as a result of mechanical stress to the fiber ferrule or thermal fluctuations of the entire system. In an attempt to overcome these factors, complex assembly and fabrication techniques are used. The fiber attachment is a complex ferrule attachment which seeks to optimize the mechanical strength of the attachment and therefore minimize the effects of fiber displacement. Finally, in order to provide a high optical coupling efficiency wide misalignment tolerances must be built in to the photodiode chip during fabrication to compensate for both displacement by the fiber attachment and deformation by temperature fluctuation.
Disclosed in U.S. Pat. No. 5,346,583 is an active alignment system for laser to fiber coupling, as illustrated in FIG. 2. The '583 patent attempts to minimize optical coupling losses by actively coupling optical energy between a source and a transmission medium. A laser 11 directs a beam 10 in the direction of a first mirror 13 and from the first mirror 13 the beam 10 is reflected to a second mirror 17 where the beam 10 is again reflected. The two mirrors are mounted on flexure elements and the flexure elements each have the capability to adjust the beam 10 in one dimension. The beam direction which is determined by the two mirrors 13 and 17 is focused by a lens 18 onto an input aperture for a waveguide contained within a modulator 20. The modulator 20 splits the beam 10 into two output beams. The two output beams are coupled at a lens 22 and focused onto a pair of fiberoptic fibers 43 and 44. Fibers 43 and 44 are each connected to electromechanical transducers 23 and 24 respectively, where the transducers have the ability to adjust the input ends of the fibers 43 and 44 in two dimensions. The active adjustment of both the mirrors (13 and 17) and the fibers (43 and 44) is accomplished by a controller 27. The controller 27 receives as input an indication of the amount of light passing through the fibers 43 and 44 from receivers 38 and 39 and supplies corrective feedback to the mirrors (13 and 17) and the fibers (43 and 44).
As discussed, present optical coupling systems use a variety of coupling schemes to obtain efficient coupling within photonics applications. However, many of these schemes use static components which are typically made of different materials and have different thermal expansion coefficients. These differences can cause optical misalignment during temperature changes, which are common in space applications. Photonics systems which are disclosed in the art and attempt to overcome misalignment problems by using active alignment control feedback techniques, typically use discrete bulk optical components and the complexity of the assembly process is increased. The greater the complexity the more assembly costs are increased and reliability decreased.
Based on techniques known in the art for photonics coupling schemes, a monolithic alignment assembly for active alignment of an optical fiber core to an optical device is highly desirable.